Proton exchange membrane fuel cell is a power generating device which converts chemical energy to electric energy directly in an electrochemical way and is thought to be the most preferred clean and efficient power generation technology. Proton exchange membrane (PEM) is a key material of proton exchange membrane fuel cell (PEMFC).
Although, the perfluorinated sulfonic acid proton exchange membranes in use currently have good proton conductivity and chemical stability at relatively low temperature (80° C.) and high humidity, they have many deficiencies, such as weak size stability, low mechanical strength, low chemical stability and the like. Since water absorption of membrane differs under different humidity, the size swelling differs due to water absorption and the size of membrane will change as the membrane changes under different working conditions. So repeated courses finally cause mechanical damage of proton exchange membrane. In addition, cathode reaction in a fuel cell will usually release plenty of substances having strong oxidability, such as hydroxyl radical, hydrogen peroxide and the like. Such substances will attack non-fluorine group of film-forming resin molecules and result in chemical degradation, damage and foaming of the membrane. Eventually, when the working temperature of the perfluorinated sulfonic acid exchange membrane is higher than 90° C., proton conductivity of the membrane descends dramatically due to rapid dehydration of the membrane and hence the efficiency of the fuel cell declines significantly. High working temperature, however, may enhance resistance of fuel cell catalyst to carbon monoxide. Moreover, all of the existing perfluorinated sulfonic acid membranes show some permeability of hydrogen or methanol, particularly in direct-methanol fuel cell permeability of methanol is very high, which turns into a fatal problem. Therefore, the fuel cell industry is faced with major issues about how to improve strength, size stability and proton conductivity at a high temperature of perfluorinated sulfonic acid proton exchange membrane, how to decrease permeability of working medium and etc.
At present, a number of solutions have been proposed to resolve these problems. For example, the Japanese Patent No. JP-B-5-75835 uses porous medium prepared by impregnation of perfluorinated sulfonic acid resin with polytetrafluoroethylene (PTFE) to increase strength of a membrane. However, since the PTFE material is relatively soft the PTFE-containing porous medium shows no sufficient reinforcement and thereby fails to overcome the above-mentioned problem. A Gore-Select series of composite membrane liquid developed by W. L. Gore uses a method of filling Nafion ion conductive liquid with porous Teflon (U.S. Pat. No. 5,547,551, U.S. Pat. No. 5,635,041, U.S. Pat. No. 5,599,614). Such membrane has high proton conductivity and relatively high size stability, but high creep of Teflon at high temperature, which results in performance reduction. The Japanese Patent No. JP-B-7-68377 also provides another method that uses porous medium prepared by filling proton exchange resin with polyolefin. But such porous medium does not have sufficient chemical durability and therefore has difficulties in the aspect of long-term stability. Furthermore, proton exchange capacity of the membrane is declined since proton conduction pathways are reduced due to addition of porous medium without proton conductivity.
In addition, the Japanese Patent JP-A-6-231779 provides another method for reinforcement that utilizes fluororesin fiber which is an ion exchange membrane reinforced by the reinforcement material fluorocarbon polymer () in the original fiber form. But it is compulsory to add a relatively amount of reinforcement material in this method; under the circumstances, it tends to be more difficult to process film and electrical resistance of membrane may probably be increased as well.
The European Patent No. EP0875524B1 discloses a technology of reinforcing nafion membrane by using glassfiber membrane prepared by applying glassfiber membrane non-woven technology. Silica and other oxides are also mentioned in the patent. But non-woven glassfiber cloth is a base material that must be used in the patent, which will greatly limit range of application of its reinforcement.
U.S. Pat. No. 6,692,858 discloses a technology of reinforcing perfluorinated sulfonic acid resin by polytetrafluoroethylene fiber, comprising mixing s perfluorinated ulfonyl fluoride resin with polytetrafluoroethylene fiber, extrusion and transformation to produce fiber-reinforced perfluorinated sulfonic acid resin. This technology cannot be applied in a continuous manner due to the time-consuming transformation process.
However, the above-mentioned technologies only simply mix porous membrane or fiber with resin. Since properties of film or fiber are greatly different from or sometimes mutually exclusive to those of film-forming resin, a gap between film-forming molecule and reinforcement article is easily generated and sometimes some pores of reinforced microporous membrane are not filled by resin. Thus, such membranes usually show high gas permeability. When a fuel cell is working, high permeability usually results in energy loss and cell damage caused by overheating.